Activation of the Raf-1 kinase cascade by coumermycin-induced dimerization

THE Raf-1 serine/threonine kinase is a key component of the MAP kinase cascade^1-3, regulating both proliferation and commitment to cell fate^4,5. Raf activation is stimulated following its transloca-tion to the plasma membrane, a process that ordinarily requires interaction with the membrane-localized GTPase, Ras-GXP^6-10. To investigate the mechanisms underlying Raf activation, we have developed a coumermycin-induced chemical dimerization method. We find that dimerization is by itself sufficient, in the absence of any membrane components, both to activate a modified Raf protein and to stimulate the MAP kinase cascade appropriately. As Ras-GTP-induced membrane localization increases the effective intracellular Raf concentration, our results indicate that homotypic oligomerization may ordinarily act to promote Raf activation in vivo.